Understanding the basic mechanisms of cell growth regulation and how alteration of such regulatory networks leads to cancer is fundamentally important for understanding cancer development and designing new strategies for treating cancer. The mTORC1 and Hippo signaling pathways are two major pathways that control cell growth and tissue/organ homeostasis. mTORC1 is a central cell growth controller which promotes cell growth by stimulating biosynthesis and inhibiting autophagy. The Hippo tumor suppressor pathway limits tissue and organ size by inhibiting proliferation and stimulating apoptosis. Dysregulation of either pathway contributes to human cancer. As such, mTORC1 inhibitors have received FDA approval for cancer treatment and there is an intensive effort in searching for drugs that can target the Hippo pathway for cancer indication. Previous works from the PI have revealed the molecular mechanisms of mTORC1 regulation by growth factors and cellular energy status via AKT and AMPK, respectively. Amino acids are arguably the most important stimuli of mTORC1. The PI has also identified Rag GTPases as critical mediators of mTORC1 activation by amino acids. Despite rapid progress in the field, key issues in amino acid signaling to mTORC1, such as the nature of amino acid sensors, remain to be solved. One major goal of this R35 proposal is to elucidate the molecular mechanism of mTORC1 activation by amino acids. The Hippo pathway is an exciting emerging field. The PI's group has made key contributions in establishing the major framework of the Hippo pathway, including identification of upstream signals, the biochemical mechanism of YAP regulation, and demonstration of the YAP-TEAD transcription module. However, fundamental issues such as regulation of core Hippo pathway components, molecular basis of YAP in promoting oncogenesis, and physiological signals that control organ size are key open questions that have yet to be answered. The overall mission of this R35 proposal is to obtain a comprehensive molecular understanding of the mTORC1 and Hippo pathways under normal physiological conditions and to elucidate how dysregulation of these pathways contributes to tumorigenesis.